The Dark-Hunter series dates back to 1998, says the lawsuit; the first in Clare’s Mortal Instruments and Shadowhunter series, City of Bones, was published in 2007.

Filed on 5 February, the lawsuit – which alleges copyright and trademark infringement and is asking for damages, lost profits and an end to infringement – lays out a host of similarities between the series.

Both Dark-Hunter and Shadowhunter books, it says, “are about an elite band of warriors that must protect the human world from the unseen paranormal threat that seeks to destroy humans as they go about their daily lives”.

“They are both given a manual on how to conduct their mission and on how to conduct themselves when dealing with other entities and species in their fictional world,” says the lawsuit.

In an exhibit, it continues: “Both series employ a line of warriors who protect the normal world from demons … In both series, a young person becomes part of the Dark-Hunters’ (or Shadowhunters’) world after being saved by a gorgeous blonde Dark-Hunter (or Shadowhunter) … They each must kill their demonic father ... Both Dark-Hunters and Shadowhunters have enchanted swords that are divinely forged, imbued with otherworldly spirits, have unique names, and glow like heavenly fire.”

According to the lawsuit, Kenyon “was alerted by some of her distressed fans” in 2006 about Clare’s planned publication “of a work that incorporated [Kenyon’s] Dark-Hunter Marks”. Kenyon asked for the Dark-Hunter references to be removed, the lawsuit says, and Clare did so, replacing it with the term “shadowhunter”. But according to Kenyon, despite assurances that use of the shadowhunter term would not be expanded, “[Clare] has persisted over time in expanding her use of the term ‘shadowhunters’ from a mere description of her protagonists, first to a tag line on the cover of her works and eventually to a complete rebranding of her works so as to be confusingly similar to the Dark-Hunter Series”.

“These substantially similar elements, coupled with [Clare’s] access to the Dark-Hunter Series, which were widely disseminated, leave little doubt that numerous substantive original elements of the Dark-Hunter Series have been copied by [Clare],” it says. “[Clare], in writing The Mortal Instruments and the Shadowhunters Series knowingly and wilfully copied the Dark-Hunter Series and original elements therein to create a work or works substantially similar to and derivative of the Dark-Hunter Series.”

Milan, a former professor of law, tweeted yesterday that “Sherrilyn Kenyon didn’t invent the idea of band of humans fighting supernatural”, and that “she’s claiming ownership over the idea that most humans are blind to a supernatural world and are given a name. She didn’t invent that.”

“Ninety-five per cent of what she is claiming are character tropes and journeys and items from our shared literary background,” wrote Milan on Twitter. “Literally both stories draw from our collective mythological history. THAT’S WHY THERE ARE SIMILARITIES … fundamental to the idea of copyright protection is that you can’t copyright ideas, only the expression of them … If you could copyright ideas, nobody would be able to write anything. Ever.”

Courtney Milan (@courtneymilan)

People talking about an "agreement" that Clare violated: If there was an agreement it should have been included as an exhibit. It wasn't.

Clare’s UK publishers Simon & Schuster and Walker Books declined to comment on the lawsuit. According to the Bookseller, Clare’s UK sales are £6.65m.

On her website, Clare writes that the idea for the Mortal Instruments series came to her when she visited a tattoo shop with a friend. “She wanted to show me that her footprints were on the ceiling in black paint. In fact, the footprints of everyone who’d worked there were on the ceiling, crisscrossing each other and making patterns. To me, it looked like some fabulous supernatural battle had been fought there by beings who’d left their footprints behind,” writes Clare. “I started thinking about a magical battle in a New York tattoo shop, and the idea of a secret society of demon-hunters whose magic was based on an elaborate system of tattooed runes just sprang into my mind.”